1. Field of the Invention
This invention relates to a method of compressing image signals. This invention particularly relates to a method of compressing image signals so that a high signal compressibility is obtained by utilizing vector quantization and encoding by orthogonal transformation or prediction encoding in combination with each other.
2. Description of the Prior Art
Image signals representing half tone images, such as television signals, are composed of enormous amounts of information, and a broad-band transmission line is required for transmission of the image signals. Such image signals involve much redundancy, and various attempts have been made to compress the image signals by restricting the redundancy. Also, in recent years, recording of half tone images on optical disks, magnetic disks, or the like has been generally put into practice. In this case, image signal compression is conducted generally for the purpose of efficiently recording image signals on a recording medium.
One of the methods of image signal compression that has heretofore been known is a method wherein vector quantization is utilized. The known method comprises the steps of (i) dividing two-dimensional image signals into blocks each comprising the signals at M.times.N number of picture elements adjacent to one another, (ii) selecting a vector that corresponds with the minimum distortion to the set of the image signals in each of the blocks from a code book comprising a plurality of vectors different from one another and prepared in advance by defining M.times.N number of vector elements, and (iii) encoding the information representing the selected vector to correspond to the block.
Since the image signals in the block as mentioned above have high correlation therebetween, the image signals in each block can be represented very accurately by one of a comparatively small number of vectors prepared in advance. Therefore, transmission or recording of the image signals can be carried out by transmitting or recording a code representing the vector, instead of the image signals themselves, and signal compression can thus be achieved. By way of example, the amount of the image signals at 64 picture elements in a half tone image of a density scale composed of 256 levels (=8 bits) is 8.times.64=512 bits. In the case where the 64 picture elements are grouped as a single block, the respective image signals in the block are expressed by a vector composed of 64 vector elements, and a code book including 256 such vectors is prepared, the amount of the signals per block becomes equal to the amount of the signals for discrimination between the vectors, i.e. 8 bits. Consequently, in this case, the amount of the signals can be compressed to 1/64.
After the image signals are compressed in the manner as mentioned above and recorded or transmitted in the compressed form, the vector elements of each of the vectors which the vector discriminating information represents are taken as reconstructed signals of each of the blocks, and the original image is reproduced by use of the reconstructed signals.
The aforesaid method of compressing image signals by vector quantization is advantageous for transmission of television signals and for other purposes. On the other hand, in recent years, an electronic image file for recording the images of very high gradation such as medical radiation images on the aforesaid optical disk or the like has attracted particular attention. For such purposes, a need exists for more efficient compression of the image signals.